Less serious answer: If you walk into a bar wearing ringmail under a travel-stained cloak and loudly ask the bartender, "What news from the North?" you may actually entice people to approach you. Likewise if you're wearing a tuxedo for no apparent reason. But nobody cares about some tired-looking guy in wrinkled khakis.

More serious answer: I've known two or three individuals in my life who were so shameless in engaging with strangers that they could legitimately go into any random bar and in short order they were the life of the party. This requires a rare type of extreme extroversion that I've often envied.

Addressing the actual question: Currently most bars are implicitly meant to be either gathering places for small groups of friends or places for opposite-sex courtship stuff. Structural changes that would motivate a more fictionesque milieu would be providing long trestle tables and a corresponding lack of private booths, more open-form, quick games to play (less billiards, more darts), "group rate" alcohol (pitchers of beer rather than individual mugs always promote sharing), and I daresay marketing the tavern as a place specifically for open socialization could help.